In a V-belt or toroidal continuously variable transmission, a target speed change ratio is generally fixed according to a load and a vehicle speed of an engine, and a real speed change ratio is controlled to follow the target speed change ratio under predetermined response characteristics.
In this type of control, when a driver depresses an accelerator pedal to accelerate, the target speed change ratio tends to vary in the increasing direction, as the engine rotation speed increases, so as to maintain the vehicle speed. As a result, the continuously variable transmission shifts down so as to reach the increased target speed change ratio.
Conversely, when the driver releases his/her foot from the accelerator pedal, the target speed change ratio varies in the decreasing direction, as the engine rotation speed decreases, so as to maintain the vehicle speed. As a result, the continuously variable transmission shifts up so as to reach the decreased target speed change ratio.
When the driver releases his/her foot from the accelerator pedal, the accelerator pedal rapidly returns to the state wherein it is not depressed. Consequently, the target speed change ratio also rapidly falls, and the engine rotation speed sharply falls for the same vehicle speed.
Some of the recent vehicles incorporate a fuel cut mechanism so that during coasting of the vehicle when the accelerator pedal is not depressed, fuel supply to the engine is cut in order to improve fuel-cost performance. When the rotation speed drops to a fuel recovery rotation speed due to fuel cut, fuel supply is resumed in order to prevent the engine from stalling.
When the above-mentioned speed change control is combined with such an engine, the engine rotation speed tends to fall to less than the fuel recovery rotation speed when the driver releases his/her foot from the accelerator pedal, and as fuel supply is immediately resumed in order to prevent the engine from stalling, improvement of fuel cost-performance due to fuel cut becomes difficult. Moreover to improve fuel cost-performance, it is desirable to prolong fuel cut time during coasting, and to lock up to the torque converter interposed between the engine and the continuously variable transmission. When fuel supply is resumed in this state, the output torque of the engine rapidly increases. This is transmitted to the vehicle as a shock, and the driver experiences a sense of discomfort.
To resolve this problem, Tokkai Hei 7-301297 for example, published by the Japanese Patent Office in 1995, discloses a method of decreasing the speed change control response sensitivity only when the driver releases his/her foot from the accelerator pedal.